The present invention relates to multichannel communication systems, and, more particularly, to a method and apparatus for attempting to seize a radio channel in a multichannel communication system. The method and apparatus of the present invention are particularly well adapted to be utilized in mobile radiotelephone systems for enabling a mobile radiotelephone station to automatically attempt to seize a radio channel for placing a telephone call-hereon.
In conventional mobile radiotelephone systems, mobile radiotelephone stations are constantly contending with one another for non-busy radio channels because of the limited number of radio channels available for such systems. These prior art radiotelephone systems are typically architectured such that all mobile stations lock on to a particular radio channel that is identified as available by the presence of a particular tone, commonly referred to as idle tone. Thus, all mobile stations continuously scan all of the radio channels for the presence of idle tone and lock to the radio channel carrying idle tone. During heavy usage periods call of the radio channels of such a radiotelephone system tend to be continuously busy, thus blocking new calls initiated by the mobile stations. As a result, all of the mobile stations are essentially constantly scanning for a radio channel carrying idle tone. If a mobile station user lifts the handset to make a telephone call under these conditions, the telephone call will not be completed and the user will be notified that the system is busy. Then, in order to reinitiate a telephone call, the user must physically place the handset back on hook and remove the handset again. Since all of the radio channels in the system tend to remain constantly busy during heavy usage periods, the user must repeatedly remove and replace the handset in order to attempt to seize a radio channel. If a radio channel becomes available, it is highly probable that several users will attempt to simultaneously seize the same non-busy radio channel. Under such conditions, at most one, but possibly none of the contending users will be successful because the respective channel seizure attempts interfere with one another. Thus, during heavy usage periods when the probability of radio channel blocking is high, not only is the user subjected to considerable inconvenience, but also system performance can be significantly degraded by the contention of multiple users for a non-busy radio channel.
The inconvenience to the user may be alleviated to some degree by providing the mobile station with the capability of automatically attempting to seize a radio channel at periodic intervals. However, if the time interval between such automatic attempts to seize a channel is substantially the same for all mobile stations, the probability of contention between multiple users is still very great. According to another approach, once a non-busy radio channel has been found, a random time delay may be provided before attempting to seize the non-busy radio channel. However, the utilization of a random time delay does not take into account whether or not the radiotelephone system is being lightly or heavily used. Thus, neither of the foregoing prior art approaches has adequately solved the problem of contention between multiple users, while also being responsive to the degree of radio channel blocking experienced in radiotelephone systems.